Solid state glass electrode

ABSTRACT

TRODE IS RUGGED HAS A RESPONSE APPROXIMATELY THAT OF A STANDARD GLASS ELECTRODE AND HAS BETTER STABILITZ OVER LONG PERIODS AND CHANGING TEMPERATURE CONDITIONS THAN PREVIOUS SOLID STATE GLASS ELECTRODES.   A SOLID STATE GLASS ELECTRODE IN WHICH A CONDUCTOR HAVING A SURFACE LAYER OF AN ELECTROCHEMICALLY ACTIVE METAL IS COATED WITH A FIRST COATING OF A MIXTURE OF A GLASS AND A HALIDE OF THE ACTIVE METAL AND A SECOND OUTER COATING OF ION SENSITIVE GLASS. THE PREFERRED ACTIVE METAL AND HALIDE ARE COPPER AND CUPROUS CHLORIDE, RESPECTIVELY. THE ELEC-

O T m 8 6 v 3 3 O 4 m v 1 I 2 B F ARNE J. PETERSEN GEORGE MATSUYAMAMarch 14, 1972 J, PETERSEN ETAL SOLID STATE GLASS ELECTRODE Filed Oct.14, 1969 United States Patent 0 3,649,506 SOLID STATE GLASS ELECTRODEArne J. Petersen, Balboa, and George Matsuyama, Fullerton, Califi,assignors to Beckman Instruments, Inc. Filed Oct. 14, 1969, Ser. No.866,249 Int. Cl. G01n 27/36 U.S. Cl. 204-195 10 Claims ABSTRACT OF THEDISCLOSURE A solid state glass electrode in which a conductor having asurface layer of an electrochemically active metal is coated with afirst coating of a mixture of a glass and a halide of the active metaland a second outer coating of ion sensitive glass. The preferred activemetal and halide are copper and cuprous chloride, respectively. Theelectrode is rugged, has a response approximately that of a standardglass electrode and has better stability over long periods and changingtemperature conditions than previous solid state glass electrodes.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to a glass electrode and a method of making the same and, moreparticularly, to a solid state glass electrode for making ion potentialmeasurements and the method of making such an electrode.

Description of the prior art The conventional glass electrode used fordetermining the ion concentration of solutions comprises a tube or stemor non-conducting glass closed at its lower end by an ion sensitiveglass membrane, usually in the shape of a bulb. Typically an internalhalf cell, such as one formed of a silver wire having a silver chloridecoating thereon, is positioned in the tube so as to contact anelectrolyte filling the lower portion of the tube and the ion sensitiveglass bulb. Such electrodes are usually somewhat fragile and aresensitive to elevated temperatures and pressures. At elevatedtemperatures the electrolyte may boil thus disrupting the operability ofthe electrode, while at high pressures the glass tubing or ion sensitivebulb may crack due to lack of structural strength. Additionally, thetypical glass electrode is difiicult to miniaturize as would be desiredto provide a micro glass electrode suitable for use in the biologicaland medical fields.

Solid state glass electrodes have been constructed in the past in whichthe electrolyte has been eliminated by coating a suitable metal wirewith a film of ion sensitive glass. Such an electrode is described indetail in an article by M. R. Thompson entitled A Metal Connected GlassElectrode, U.S. National Bureau of Standards Journal of Research, vol.9, 1932, pp. 833 to 852. Another form of solid state glass electrode isdescribed in British Pat. 1,018,024. This electrode is made bysuperficially oxidizing a copper wire and thereafter applying a coatingof ion ice sensitive glass to the oxidized surface. It is stated in thepatent that such an electrode has greater stability than that of anelectrode in which a metal wire is directly coated with a layer of ionsensitive glass. By stability, reference is made to the ability of aglass electrode to produce a substantially constant millivolt responseover time and changing temperature conditions.

SUMMARY OF THE INVENTION It is the principal object of the presentinvention to provide an improved solid state glass electrode.

Another object of the invention is to provide a solid state glasselectrode which produces a stable electrical output over time andvarying temperature conditions without loss of sensitivity.

According to the principal aspect of the present invention, we havediscovered that if an intermediate layer of a mixture of glass and ahalide of an electrochemically active metal is provided between themetal contact or conductor and the outer coating of ion sensitive glassof a solid state glass electrode, the electrode will be rugged, and willproduce a relatively stable electrical output over time and undervarying temperature conditions without a loss in sensitivity to changingion concentrations of test solutions.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary sectional viewof the sensing end of one electrode constructed in accordance with thepresent invention, shown greatly enlarged;

FIG. 2 is a fragmentary sectional view of the sensing and of anotherelectrode embodying the features of the invention, also shown greatlyenlarged; and

'FIG. 3 is a fragmentary, enlarged sectional view of a furtherembodiment of the invention in the form of a combination glass-referenceelectrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the invention, wehave found that a solid state glass electrode may be formed having goodresponse and electrical stability characteristics by providing between aconductor having a surface layer of an electrochemically active metaland the outer layer of ion sensitive glass an intermediate layer orcoating comprised of a glass having a halide of such active metalincorporated therein. Examples of suitable electrochemically activemetals which may be utilized in the electrode of the present inventionare copper, silver, cobalt and cadmium while suitable halides of suchmetals which may be used are chloride, iodide and bromide. As willappear later, the preferred electrode of the present invention is onehaving an in ternal metal contact of copper, an intermediate layerformed of a mixture of ion sensitive glass and cuprous chloride, and anouter layer or coating formed of pH sensitive glass. It is to beunderstood, however, that the present invention is not limited to theuse of hydrogen ion sensitive glasses, but may utilize other forms ofion sensitive glasses, such as cationic and sodium ion sensitiveglasses.

Referring now to FIG. 1 of the drawing, there is shown one embodiment ofthe present invention comprising an elongated metal conductor having anupper portion generally designated 12 and a lower portion generallydesignated 14. Disposed upon the upper portion 12 of the metal conductoris an electrical insulating coating 16 and disposed about and sealed tothe lower portion 14 of the conductor is a coating 18 of glass mixedwith a halide of a metal the same as that of the conductor. Sealed toand coating the layer 18 and the terminal end of the insulating coating16 is an ion sensitive glass coating which forms the ion sensitivemembrane of the electrode.

The metal conductor 10 may be formed of an electrochemically activemetal, such as any one of those mentioned above, and preferably has acoeflicient of thermal expansion close to that of the electricalinsulating layer 16. In the preferred form of the invention, theconductor 10 is a metal wire while the electrical insulating coating 16may be a nonconductive glass tube or capillary which is sealed to thewire 10. The upper portion 12 of this Wire may be connected to asuitable cable (not shown) for coupling the electrode to a highimpedance amplifier, such as a conventional pH meter.

For reasons to be explained later, the end 22 of the lower portion 14 ofthe metal conductor is rounded by any convenient method such as bygrinding, bending the end of the wire upwards to leave the lowest mostportion a smooth curve, or by heating the end to form a smooth sphericalbead.

The glass employed int he coating 18 may be .a nonconductive glass or anion sensitive glass, such as the same ion sensitive glass which isemployed to form the outer layer or membrane 20. The use of an ionsensitive glass in the coating 18 has the advantage that the resultingelectrode has a lower electrical resistance than when nonconductiveglass is used in the coating.

In general, the method of manufacturing the electrode illustrated inFIG. 1 comprises the steps of coating the upper portion 12 of the metalconductor 10 with the insulating coating 16, sealing a coating or layercomprising a glass mixed with a halide of the metal conductor to thelower portion 14 of the conductor and, finally, coating the layer 18with an ion sensitive glass.

In more detail, the insulating coating 16 for the conductor 12 may beformed by surrounding the conductor 10, except for its lower portion 14,by a capillary glass tube which has a coeflicient of thermal expansionsimilar to that of the conductor 12. The tube is then heated so that itwill shrink or collapse upon cooling, and thus seal against the outersurface of the conductor 10. Thereafter, a glass, which may be ionsensitive glass, is mixed with a halide of the metal conductor and isapplied to the exposed lower portion 14 of the conductor. The coating 18may be applied by any convenient method. One of such methods is themixing of powdered glass and salt, depositing such mixture upon theexposed end 14 of the conductor, fusing the mixture in a flame and thenallowing the coating thus formed to cool and solidify. A second methodis the mixing of the powdered glass and metal halide, heating themixture until it is molten, dipping the lower portion 14 of theconductor into the molten mixture and then allowing the same to cool. Byeither of these two methods, it has been found that the salt employed inthe coating 18 is uniformly distributed throughout the glass utilized inthe coating. The weight percentage of salt to glass in the prefusionmixture then forms the coating 18 is preferably within the range ofabout 0.05% to 2.0%. This ensures a sufficient amount of salt in thecoating to make the coating a good electrical conductor and thus capableof functioning as an internal half cell.

The ion sensitive glass membrane or coating 20 is then formed over thecoating 18 by dipping the coated lower portion 14 of the conductor 10into a molten bath of ion sensitive glass up to a level slightly abovethe lower end 21 of the insulating layer 16. The end of the conductorshould remain disposed in the molten ion sensitive glass for a timesufiicient to wet the coating 18. Then the conductor is removed from themolten bath of ion sensitive glass and the glass coating formed on thelayer 18 is then allowed to cool and solidify. It has been found that byrounding the end 22 of the conductor as discussed above the glass 20,upon solidification, pulls away from the end of the conductor thusleaving an extremely thin membrane or film, thus minimizing theelectrical resistance of the electrode.

In practice, it is diflicult to find suitable insulating materials forthe conductor 12 which have coefiicients of thermal expansion matchingthat of the conductor so that the insulating coating will not crack orotherwise fail over a wide range of temperatures. To overcome thisproblem, the embodiment illustrated in FIG. 2 is provided. In thisembodiment, there is provided an elongated metal conductor 30,preferably a metal wire, having an upper portion designated 32 and alower portion designated 34. Disposed about and sealed to the upperportion of the conductor 30 is an electrical insulating coating 35. Theexposed lower portion 34 of the conductor is coated with a layer ofmetal 36 upon which is sealed a second coating 38 which is comprised ofa glass mixed with a halide of the metal coating 36, like the layer 18in the embodiment illustrated in FIG. 1. Covering the coating 38 is anion sensitive glass membrane coating 40.-

The conductor 30 is for-med of a metal, such as platinum, for whichthere is readily available electrical insulating materials having acoefiicient of thermal expansion closely matching that of the metal. Forexample, lead glasses, such as Corning 0120 or Kimble KG-l2, havecoefficients of thermal expansion closely matching that of platinum. Byusing such glasses as the insulating coating 36 for the wire 30, theresulting electrode may be subjected to a wide variety of temperatureconditions without cracks forming in the glass coating. Since platinumis not a sufiiciently electrochemically active metal to be utilized fordirect metal contact to the glass-salt coating 38, the lower end 34 ofthe conductor 30 is plated with a more active metal, such as thosediscussed above in connection with the metal which forms the conductor10. The coating 38 of glass and salt is identical to that described forcoating 18 in FIG. 1 with the salt being chosen to correspond to theplated metal 36 rather than the conductor 30. For example, if silver isused as the metal coating 36, it is preferred that silver chloride beused as the salt in the coating 38, or if copper is the plating metalthen cuprous chloride should be utilized in the coating 38.

To construct the electrode illustrated in FIG. 2, the upper portion 32of the platinum wire 30 is surrounded by a capillary tube of a glass,such as Corning 0120 or Kimble KG-l2 or the like, which is scalable toplatinum. The tube is heated to a sufiiciently high temperature to causeit to shrink and seal onto the platinum wire 30 upon cooling to roomtemperature. Thereafter, the lower portion 34 of the platinum wire isplated with an active metal by electroplating or dipping the end 34 ofthe wire in an oxide of the active metal and fusing the same in a flame.A halide of such metal is then mixed with a suitable glass and appliedto the metal coated platinum wire in the manner discussed above. The ionsensitive glass layer 40 is then formed over the coating 38 ashereinbefore described. Hence, at the lower portion of the conductorfour layers are sealed each to the layer or layers adjacent to itthereby providing a rugged, compact electrode having the ability towithstand high temperatures and pressures.

Reference is now made to FIG. 3 wherein another embodiment of theinvention is illustrated in the form of a combination electrode, thatis, an electrode which has a sensing portion as well as a referenceportion. The sensing portion designated '50 has the same construction aseither of the electrodes illustrated in FIG. 1 or 2. As shown, theelectrode 50 includes a metal conductor 52 having an upper portion 54and a lower portion 56. An

electrical insulating material 58 covers the upper portion 54, while therequisite layers are applied to the lower portion 56 as discussed abovein connection with FIGS. 1 and 2. Disposed about the insulating layer 58is a tube 59 of nonconductive material, such as a nonconductive leadglass. This tube is spaced from the electrical insulating layer 58 sothat an annular space 60 is formed between the walls of the tube and theinsulating layer. The space 60 is filled with a reference electrodeelectrolyte 61 which may be any one of the types normally used and wellknown to those skilled in the art. An internal half cell comprising asilver wire 62 coated with silver chloride is disposed in the space 60and wound about the insulated conductor 52. One end 64 of the wire 62extends beyond the electrode and is adapted to be connected to a pHmeter [not shown] together with the conductor 52. An elongatednonconductive glass capillary tube 66 surrounds and is spaced from theinsulated conductor 52. This tube extends from within the space 60 tojust adjacent the lower portion 56 of conductor 52. The tube 66 isspaced slightly from the insulated conductor so that a narrow annularpassage 68 is formed therebetween. This passage is occupied :bylongitudinally extending fibers 70, such as quartz roving or asbestosfibers, which form the liquid junction for the electrolyte within thechamber 60. An opening 74 in the tube 59 is provided so that electrolytemay be selectively added to the chamber 60 as electrolyte is dissipatedthrough the liquid junction. A plug 76 at the upper end of the tube 59closes the upper portion of the chamber 60.

To manufacture the combination electrode illustrated in FIG. 3, thesensing portion may be made as already described above. In addition, thecapillary tube 66 containing quartz fibers 70 or the like is disposedabout the insulated conductor 52 and is heated to collapse about thefibers and seal the same into position between the inner wall of theglass tube 62 and the insulated conductor 52. Then the lower end of theglass tube 59 is sealed to the glass capillary 66. The internal halfcell 62 is then mounted in the reservoir 60 and the upper end of theassembly is closed by the plug 76. A combination electrode has beensuccessfully constructed as shown in FIG. 3 utilizing a glass tube 62having an outer diameter of about 3.5 mm. and with the capillary tube 66having an outer diameter of less than about 1 mm.

Four electrodes constructed in accordance with the teachings of thepresent invention have been tested, together with a conventional glasselectrode and an electrode as disclosed in the aforementioned Britishpatent, in various test solutions, at different temperatures and over aperiod of some three days. Each electrode was coupled with a standardreference electrode to a conventional pH meter. The results of thesetests appear in Table I below.

TABLE I A B C Cul% CutClz Standard glass electrode (Du-Oxide pH 4 pH7.41 pH 4 pH 7.41 pH 4 pH 7.41

(vs-1% CdCl2 pH 4 pH 7.41

Ag1% AgClz pH 4 pH 7.41

Time

As will be noted, the above Table I is divided into six columns,designated by the letters A, B, C, D, E and F, one for each electrodetested. The terms pl-I 4 and pH 7.41 in the table have reference to thehydrogen ion concentrations of two sets of test solutions to which eachof the electrodes were subjected. The terms and 40 under each of thecolumns indicate the temperature in degrees centigrade of the testsolutions. The positive and negative figures under each temperaturecolumn are the actual millivolt output readings of the electrodes. Theleft hand column entitled time has reference to a period of three daysover which the electrodes were tested in the test solutions. The figuresin the horizontal rows designated X indicate the deviation in rnillivoltreadings of the electrodes over the three day period of time that theelectrodes were tested, while the figures in the horizontal rowsdesignated Y indicate the maximum change in rnllivolt response of theelectrodes when shifted from the 25 test solutions to the 40 testsolutions. The test data under column A are for an electrode like thatillustrated in FIG. 2 in which the wire was formed of platinum, themetal coating 36 was formed of copper, the coating 38 was formed of amixture of hydrogen ion sensitive glass and 1% by weight of cuprouschloride, and the outer coating 40 was for-med of the same glass as usedin the coating 38. The test data appearing under column B are for aconventional glass electrode having an ion sensitive bulb formed of aglass which was the same as the glass utilized in the layers 38 and 40for electrode A. The data under column C are for an electrodeconstructed in accordance with the teachings of the aforementionedBritish patent, employing the same ion sensitive glass as in the otherelectrodes discussed herein. The data appearing under columns D, E and Fare for electrodes similar to electrode A but employing a ditferentactive metal and salt. In the electrode designated D theelectrochemically active metal was cobalt and the layer 38 a mixture ofhydrogen ion sensive glass and 1% by weight of cobalt chloride. In theelectrode designated E the active metal was cadmium and the layer 38 amixture of the aforementioned ion sensitive glass and 1% by weightcadmium chloride. In the electrode designated F the active metal wassilver and the layer 38 a mixture of ion sensitive glass and 1% byweight silver chloride.

The theoretical response for any of the electrodes dis cussed abovewould be 201.4 millivolts at 25 and 207.8 millivolts for 40. It is seenfrom the data appearing in Table I that all the electrodes producedclose to theoretical response. In comparing the stability of theelectrodes, it is seen that the copper-1% cuprous chloride electrode hada stability in electrical output over a three day period of time andchanging temperature conditions closely approximating that of thestandard glass electrode B. While the stability of the electrodesdesignated D, E and F was not quite as favorable as the electrodedesignated A, it is seen that the former electrodes were substantiallymore stable than the electrode constructed in accordance with theteachings of the aforementioned British patent, the data for whichappears under column C. Thus, by the provision of an intermediate layerformed of a glass having a halide of active metal incorporated thereinbetween the active metal surface layer and ion sensitive coating,substantially greater stability is achieved than that by the prior artBritish electrode, and the stability of the pre ferred electrode of thepresent invention, i.e., the electrode utilizing copper and 1% cuprouschloride in the coating 38, has a stability close to that of a standardglass electrode.

Although several embodiments of the invention have been disclosed hereinfor purposes of illustration, it will be understood that various changescan be made in the form, details and arrangement of the various partsand in the materials and proportions of the various materials in suchembodiments without departing from the spirit and scope of the inventionas defined by the appended claims.

What is claimed is:

1. A glass electrode for measuring the ion concentration of solutionscomprising:

an elongated metal conductor;

said conductor having at one end thereof a surface layer of anelectrochemically active metal;

a first coating covering said layer, said coating comprising a glasshaving a salt uniformly distributed therethrough, said salt being ahalide of said active metal; and

a second coating of ion sensitive glass covering said first coating.

2. An electrode as set forth in claim 1 wherein:

said surface layer of active metal is provided by a separate coatingcovering said one end of said conductor;

said conductor being formed of a metal other than said active metal; and

an insulating coating covering said conductor except for said one endthereof and having a coefficient of thermal expansion closely matchingthat of said conductor.

3. An electrode as set forth in claim 2 wherein:

said conductor is a platinum wire; and

said insulating coating is glass.

4. An electrode as set forth in claim 1 wherein said active metal isselected from the group consisting of copper, silver, cobalt andcadmium.

5. An electrode as set forth in claim 1 wherein said salt is a chloridesalt.

6. An electrode as set forth in claim 1 wherein said active metal iscopper and said salt is cuprous chloride.

7. An electrode as set forth in claim 1 wherein:

the end of said conductor is rounded.

8. An electrode as set forth in claim 1 wherein:

said glass of said first coating is the same as said ion sensitiveglass.

9. A glass electrode for measuring the ion concentration of solutionscomprising:

an elongated metal conductor;

said conductor having at one end thereof a surface layer of anelectrochemically active metal;

a first coating covering said layer, said coating comprising a glasshaving a salt uniformly distributed therethrough, said salt being ahalide of said active metal;

a second coating of ion sensitive glass covering said first coating;

a glass insulating coating covering said conductor except for said oneend thereof;

a first glass tube surrounding said conductor adjacent to said one endthereof, said tube being closely spaced from said glass insulatingcoating to define a restricted annular passage therebetween;

at least one strand of inert material extending the length of saidpassage and being retained therein by the opposed surfaces of said firstglass tube and said glass insulating coating so as to provide a liquidjunction;

a second glass tube surrounding said conductor adjacent to the other endthereof, the end of said second glass tube adjacent to said first glasstube being fused thereto, said second glass tube being spaced from saidglass insulating coating to provide an annular electrolyte reservoirtherebetween;

an internal half cell positioned in said reservoir; and

means sealing the other end of said second glass tube to said glassinsulating coating.

9 10 10. An electrode as set forth in claim 9 wherein: 2,755,243 7/1956Beckman et a1. 204-195 said conductor is a metal wire; and 3,103,480 9/1963 Watanabe et a1 204-195 said first glass tube is an elongatedcapillary tubing. FOREIGN PATENTS References Cited 5 587,967 5/1'947Great Britain 204195 UN T STATES PATENTS GERALD L. KAPLAN, PrimaryExaminer 3,306,837 2/1967 Riseman et a1. 204195 3,498,901 3/1970 Metz eta1. 204-195 CL 3,049,118 8/1962 Arthur et a1. 204195 X 10 6559

